Viruses means venom or poisonous fluid. These were discovered by Ivanowsky. Viruses are composed of nucleoproteins. Structurally a virus j consists of two major parts :(i) Nucleoid : It is the core of nucleic acid and is infective part of the virus. It is the genetic material in the form of either DNA or RNA. The nucleic acid of viruses can be of four types (a) Double stranded DNA (b) Single stranded DNA (c) Single stranded RNA (d) Double stranded RNA.(ii) Capsid : The nucleoid is covered by a protein coat called capsid. It is made up of specifically arranged sub-units called capsomeres. (iii) Envelope : Some animal viruses like HIV have loose covering over the capsid called envelope. It is j made up of proteins, lipids and carbohydrates. The proteins of capsid or envelope act as antigens.(iv) Enzymes : A few proteins in some viruses are enzymetic in functions, e.g. lysozyme in bacteriophage. Viruses causes serious disease in humans, animals and plants like mumps, small pox, herpes, influenza and AIDS etc.